1. Field of the Invention
The present invention relates to a power tool using an AC motor. More particularly, the present invention relates to a power tool having controller for controlling a rotation speed of a motor with a conduction angle of a semiconductor device and to a power tool having controller for controlling a rotation speed of a motor with a conduction angle of a semiconductor device.
2. Description of the Related Art
In a power tool, it is important to control a rotation speed of the motor to not fluctuate even if a load to be applied to the motor is changed. For this reason, it has widely been carried out to detect the rotation speed of the motor by a rotation speed detection unit and to monitor a difference between a result of the detection and a set rotation speed, thereby performing a rotation control. For example, in JP-2004-194422-A, an AC motor is used as the motor of the power tool. When the AC motor is used, a signal output from a rotation speed detection unit is fed back to a signal for setting a conducing angle of a triac, thereby carrying out a constant rotation control for maintaining a rotation speed of the motor to be constant.
In order to increase/decrease the rotation speed of the motor, generally, a conduction angle of a semiconductor device such as a triac is changed as described in JP-2004-194422-A. The conduction angle is expressed in % within an angle range (0° to 180°) from a phase angle at which the triac is turned ON to a zero-cross point. In order to protect the motor, a current flowing to the motor is detected and the motor is stopped when a preset overcurrent set value is exceeded. An exemplary conventional control method will be described with reference to FIG. 5.
FIG. 5 illustrates a motor rotation control circuit according to the conventional art. An AC power supply 101 has a single phase of 100 V at 50 Hz or 60 Hz, for example, and an alternating current is ON/OFF controlled by a switch 102. A phase angle for starting a conduction is controlled by a triac 127 so that a rotation speed of a motor 103 is controlled. A shunt resistor 140 is inserted for detecting a current flowing to the motor 103 and a custom IC 123 measures a voltage applied to both terminals of the shunt resistor 140, thereby detecting a current value. The custom IC 123 includes an overcurrent detection circuit, which is not shown. If it is decided that an overcurrent flows to the motor 103, a supply of a power to the motor 103 is blocked. A diode 116, a resistor 117 and an electrolytic capacitor 119 are power circuits for carrying out a half-wave rectification and a direct current which is generated is supplied to the custom IC 123.
A resistor 131 and variable resistors 132 and 133 have resistance values for setting the rotation speed of the motor 103, and a voltage (set voltage) set by the variable resistor 132 is input to a positive side of a comparator 123a in the custom IC 123. The motor 103 is provided with a rotation speed sensor 106 for detecting the rotation speed, and an output (feedback voltage) of the rotation speed sensor 106 is input to a negative side of the comparator 123a. The comparator 123a generates a trigger signal based on a difference between the set voltage and the feedback voltage, and the trigger signal is input to the triac 127 through a resistor 128 to control a conduction angle of the triac 127 so that a voltage to be applied to the motor 103 is regulated.
In the method of detecting an overcurrent according to the conventional art, it is necessary to insert the shunt resistor 140 in a circuit for supplying a current to the motor. For this reason, the current flowing to the motor always increases a power consumption in order to flow to the shunt resistor 140. It is necessary to incorporate, in the custom IC 123, a circuit for detecting a voltage applied to both terminals of the shunt resistor 140 and deciding whether an overcurrent flows or not.
In a power tool, it is important to control the rotation speed of the motor to not fluctuate even if a load to be applied to the motor is changed. For this reason, the rotation speed of the motor is detected by a rotation speed detection unit and a difference between a result of the detection and a set rotation speed is monitored to carry out a feedback control. In a power tool using an AC motor, a difference from the set rotation speed which is detected is reflected by a change in a conduction angle of a triac so that the rotation speed of the motor is maintained to be constant. The conduction angle is expressed in a percentage (%) of an angle range (0° to 180°) from a phase angle at which the triac is turned ON to a zero-cross point. In the power tool using the AC motor, the motor is frequently turned ON/OFF. Therefore, it is important to start the motor quickly and stably. Although not a control in the power tool, JP-H06-254440 discloses a method of starting an AC motor without an overshoot or an undershoot when a setting state is brought after starting in a rotor rotation control of a centrifugal separator.
The prior art in which a difference between a target rotation speed of the AC motor and an actual rotation speed is fed back to a conduction angle of a triac can be well applied to a control in the case in which the motor is rotated at a certain rotation speed. However, the inventors found that there is the following problem if the prior art is exactly applied to a control in the starting operation of the motor. The state will be described with reference to FIG. 8.
FIG. 8 illustrates a starting characteristic of the motor. An axis of ordinate indicates a rotation speed (r.p.m) of the motor and an axis of abscissa indicates an elapsed time (second). In the drawing, a curve shown in a solid line represents an ideal accelerating situation from a starting operation in the motor to a target rotation speed. When a conduction angle of a triac is set to be slightly great in order to prevent a delay of a responsiveness of the motor in the starting operation of the motor, an acceleration is carried out after starting the motor and a target rotation speed is once exceeded like a curve shown in a dotted line to cause an overshoot, and the target rotation speed is approximated while an undershoot and the overshoot are then repeated. In order to prevent the overshoot or the undershoot, it can also be proposed to cause a responsiveness of a rotation speed control to be poorer (slower) than that of the motor. In that case, the overshoot can be prevented. However, the acceleration of the motor is made slow like a curve shown in a dotted line on a lower side, that is, a so-called slow start is carried out. Consequently, a long time is taken for the starting operation.
Also in the technique described in JP-H06-254440, there is eliminated the overshoot or the undershoot when a setting state is brought after the starting operation. However, the AC motor to be used is an induction motor and a motor current is regulated to control an angle of ignition of a phase control device for controlling a current to be given to the induction motor (which is included in a motor driving circuit), thereby eliminating the overshoot. In the technique described in JP-H06-254440, however, an alternating current having both frequencies including a slip frequency and a synchronizing frequency is used to carry out the control in the starting operation, and furthermore, the angle of ignition is switched. Therefore, a premise is different from that in a method of controlling an AC motor to be started and accelerated without changing a frequency as in an AC motor such as a commutator motor for carrying out a control with only the conduction angle of the triac. Accordingly, the technique described in JP-H06-254440 cannot be exactly applied to the AC motor such as the commutator motor.